Addition-type polyimides, derived from end-capped polyimide oligomers, typically undergo thermal cross-linking or chain extension to form a crosslinked polyimide resin. Addition-type polyimides provide suitable matrix materials for high temperature polymer matrix composites due to their desirable heat resistance, desirable mechanical properties, desirable tribilogical properties, high chemical resistance and high radiation resistance. However, the processibility of given polyimides are limited and the range of properties are limited to the particular type of polyimide fabricated.
High temperature parts, such as gas turbine engine components are typically fabricated by a hand lay-up method. The hand lay-up method typically includes positioning a prepreg fiber onto a mold and pouring a liquid resin onto the fiber. The curing typically takes place at room temperature and the blend is rolled to work out any air bubbles and to fully distribute the resin. In addition, the manipulation of the resin to remove air bubbles and to distribute the resin may result in damage to the fibers making up the composite. This method suffers from the drawback that the processing method is labor intensive and suffers from high costs. Alternative methods, such as resin film infusion (RFI), are desirable techniques due to the decreased labor costs related to performing RFI and the reproducible parts that may be achieved. The curing typically takes place at elevated temperatures in an autoclave and the cure is done in a vacuum bag under high pressure (typically 100-200 psi) in order to make the resin flow and remove entrapped air and condensable gases. However, conventional polyimide oligomers lack the processibility required for fabrication of parts using RFI. For example, known polyimides typically include a high melting or low molecular weight powder, but lack the flexibility of the combination of melting temperature and molecular weight that is desirable for processing techniques, such as RFI.
Currently, addition-type polyimides are used either as a monomeric solution (e.g., PMR-15 monomeric solutions) which reacts in a 2 step fashion to form a crosslinked system or as preimidized powders which melt prior to crosslinking to again form a crosslinked system. Monomeric solutions of prepolymer polyimides typically include a diamine, a dianhydride and an end blocking agent having a crosslinkable group. PMR-15, for example, is a reaction product of monomethyl ester of 5-norbornene 2,3-dicarboxylic acid, dimethyl ester of 3,3′,4,4′-benzophenone tetracarboxylic acid and 4,4′ methylenedianiline (MDA). PMR-15 is a material that has found extensive use in gas turbine engine component manufacture. However, the partially unreacted solutions of PMR-15 include MDA, which is a known carcinogen and is a known liver and kidney toxin. Fully reacted under cured PMR-15 compound mixtures no longer contain MDA and are less hazardous to handle. Nonetheless, while the properties of PMR-15 are suitable for use in the fabrication of higher temperature gas turbine engine parts, the use of MDA during the fabrication of the polyimide resin significantly increases costs and processing complexity.
What is needed is a polyimide prepolymer and crosslinked polyimide system that includes properties that may be tailored to particular applications and are fabricated by methods that include less hazardous chemicals. Further, what is needed is a method for fabricating polyimide materials that reduces or eliminates the requirement for hazardous and/or carcinogenic materials.